An electrophoretic display (EPD) is a non-emissive bi-stable output device which utilizes the electrophoresis phenomenon of charged pigment particles suspended in a dielectric fluid to display graphics and/or alphanumeric characters. The display usually comprises two plates with electrodes placed opposing each other. One of the electrodes is usually transparent. The dielectric fluid which includes a suspension of electrically charged pigment particles is enclosed between the two plates. When a voltage potential is applied to the two electrodes, the pigment particles migrate toward the electrode having an opposite charge from the pigment particles, which allows viewing of either the color of the pigment particles or the color of the dielectric fluid. Alternatively, if the electrodes are applied the same polarity, the pigment particles may then migrate to the one having a higher or lower voltage potential, depending on the charge polarity of the pigment particles. Further alternatively, the dielectric fluid may have a clear fluid and two types of colored particles which migrate to opposite sides of the device when a voltage potential is applied.
There are several different types of EPDs, such as the conventional type EPD, the microcapsule-based EPD or the EPD with electrophoretic cells that are formed from parallel line reservoirs. EPDs comprising closed cells formed from microcups filled with an electrophoretic fluid and sealed with a polymeric sealing layer are disclosed in U.S. Pat. No. 6,930,818, entitled “Electrophoretic Display and Novel Process for Its Manufacture”, issued on Aug. 16, 2005 to the assignee hereof, the entire contents of which is hereby incorporated herein by reference for all purposes as if fully set forth herein.
Electrophoretic type displays are often used as an output display device for showing a sequence of different or repeating images formed from pixels of different colors. Because the history of voltage potential levels applied to generate the images is different for each pixel, the voltage potential stress on each pixel of the display is typically different. These differences from pixel to pixel, in general, lead to long term issues with image uniformity. Although attempts have been made previously to alleviate such problems with waveforms that have no DC bias or by use of clearing images to reduce non-uniformity, neither of these approaches provides a practical solution to such problems for the long term.